This invention relates generally to the vulcanization of active halogen-containing elastomers such as copolymers and the use of curing agents or accelerators which control reaction time so as to allow the elastomer to flow into a mold while still maintaining an acceptable total cure time and surface quality of the vulcanized elastomer.
Ideally, an elastomer cure process would follow a rheometer cure curve wherein (i) the elastomer is heated rapidly as it is introduced into a warm mold and undergoes a limited amount of vulcanization or cross linking, (ii) the elastomer remains in a plastic, flowable state as it fills the mold, (iiI) after filling the mold, which should take less than one minute, the elastomer would ideally cure instantaneously by allowing all available or required cross linkage to occur at once so that the mold can be opened and the finished parts removed. As a practical matter, no elastomer can be cured in such an ideal manner, but improvements are constantly sought.
One problem that needs to be solved in formulating an improved cure system is finding the desired combination that will allow enough time for the elastomer to fill the mold while in the flowable state without unduly prolonging the total cure time. While acceptable cure systems have been developed for many commonly used elastomers, an efficient cure system has not been developed for halogen-containing elastomers that does not result in poor surface quality of the cured elastomer. For example, vulcanizing ingredients such as sodium stearate, which are commonly used to cure halogen-containing elastomers, result in blemishes visible at 40X magnification. A smooth blemish-free surface is essential for satisfactory performance of elastomeric seals.
The term "halogen-containing elastomers" is defined as those polymers and copolymers which contain a halogen atom as a substituent on a recurring unit in the polymer structure through which vulcanization can be achieved at least in part. Among the elastomers containing active halogens that are commercially available or known are included the following: butyl-isobutylene-isoprene copolymers containing at least one halogen selected from the group consisting of chlorine, bromine, and iodine such as described in U.S. Pat. Nos. 2,631,984, 2,681,899, 2,698,041, 2,700,997, 2,720,479, and 2,732,354, as well as those described in Rubber World, vol. 138, page 725 (1955) and Ind. Eng. Chem., vol. 47, page 1562 (1955), to which reference is made for further details, and the disclosures of which are hereby incorporated by reference; acrylates, such as copolymers of an acrylate as represented by ethyl acrylate and vinyl chloroethyl ether, 2-chloroethyl acrylate, vinyl haloacrylates typified by vinyl butyrate, and the like, wherein the halogens are as previously indicated and at least one is present on the alphacarbon of the acid involved and where additional comonomers may be employed along with the acrylate and the halogen-containing comonomer may be as described in U.S. Pat. No. 3,201,373, to which reference is made for further details, and the disclosure of which is hereby incorporated by reference; polyesters wherein halogens are present in the polymers through condensation of suitable amounts of appropriately substituted acids or glycols; halogenated polyethylene wherein about one atom of halogen is present per 6-7 atoms of carbon in the chain and some of the halogens are present in the form of -SO.sub.2 X where X represents a halogen as previously defined; and copolymers of vinylidene chloride.
Various agents have been used or suggested for the curing of halogen-containing elastomers. Among the curing agents which have been suggested are ammonia and various primary and secondary amines. Unfortunately, for most purposes, ammonia and amines react too fast as curing agents in many uses. This is particularly true of elastomers based on acrylates and vinyl haloacrylates.
Improved curing systems for halogen-containing elastomers are known and relate to the use of curing agents such as the following: ammonium salts, ammonium salts plus alkyl halides, and ammonium salts plus alkaline earth metal oxides. Several such systems are shown in U.S. Pat. Nos. 3,324,088 and 3,458,461, which relate to the use of ammonium salts and alkali metal salts, respectively, and to which reference is made for further details and the disclosures of which are hereby incorporated by reference. While these curing systems offer several advantages over former systems, particularly ammonia and amines, there exists a continuing need for improved curing systems which are (i) effective with a variety of halogen-containing elastomers, (ii) capable of variation to control curing rates, (iii) improve surface quality, (iv) provide economic advantages, and (v) otherwise overcome deficiencies of the known systems.
Uses of halogen-containing elastomers vary widely and are influenced to some extent by the chemical nature of the elastomers, with the peferred use for a particular elastomer being one wherein maximum advantage is taken of the special chemical properties of the elastomer. For example, copolymers of an acrylate (such as acrylonitrile) and a vinyl-haloacrylate form oil-resistant molded products useful in the making O-rings, oil seals, gaskets and the like. It is also noteworthy that with many of the previously used curing agents it is necessary to post-cure the vulcanized article for an extended period of time at a temperature of about 150.degree. C. (about 302.degree. F.) in order to fully develop the physical properties desired in the vulcanizate.
It is therefore an object of this invention to provide upon known systems for curing reactive halogen-containing elastomers.
It is a further object of the present invention to provide a curing system which is economical and effective from the standpoint of improved elastomer flow in the mold and rate of cure while maintaining good surface quality.
Another object of this invention is the elimination of postcuring operations.
It is also an object of the present invention to improve the surface quality of the resulting elastomeric product.
In accordance with the present invention, the properties of halogen-containing elastomers are improved by incorporating into the elastomer before vulcanization from about 0.05 to about 3 parts by weight of dimercaptothiodiazole (DMTD), and from about 0.5 to about 5 parts by weight of a dithiocarbamate type accelerator per 100 parts of elastomer. Although the curing process of this invention is usable with any of the halogen-containing elastomers previously named, it is particularly valuable for curing those elastomers containing haloacrylate groups. Zinc di-n-butyldithiocarbamate and dimercaptothiodiazole in acrylic elastomers containing active chlorine is a preferred combination. The process of the present invention comprises mixing the unvulcanized elastomer with dimercaptothiodiazole and the dithiocarbamate accelerator, with or without N-(cyclohexylthio)-phthalimide vulcanization inhibitor and other additives. The compounding can be done on a rubber mill or internal mixer by conventional procedures. The compounded elastomer is then cured at an elevated temperature, typically in a heated mold.
Additional benefits and advantages of the present invention will become apparent upon a reading of the detailed description of the preferred embodiments taken in conjunction with the accompanying examples.